Field of the Invention
The present invention relates to an optical observation unit, in particular a medico-optical observation unit having an illumination apparatus for illuminating an observation object. The medico-optical observation unit can in particular be a surgical microscope or an endoscope. The invention additionally relates to a method for ensuring an unchanging illumination intensity when changing the illumination from a first color temperature to a second color temperature by inserting or removing a spectral filter in or from the illumination beam path of an optical observation unit.
Description of the Related Art
U.S. Pat. No. 7,443,579 B2 and DE 10 2007 026 044 B3 disclose illumination apparatuses for surgical microscopes which have either a xenon gas discharge lamp or a halogen incandescent lamp as a white-light source. Xenon gas discharge lamps and halogen incandescent lamps both emit what is referred to as white light, but they differ in terms of the color temperature of said light emitted by them. While xenon gas discharge lamps emit light with a correlated color temperature of about 6000° K (often also referred to as “cold white light”), halogen incandescent lamps emit light with a correlated color temperature of about 3000° K (often also referred to as “warm white light”).
When conducting ophthalmic surgeries such as cataract surgeries where the lens of the eye is removed, it may be advantageous, depending on the type of defect, to illuminate the operating site in the eye with a correlated color temperature of 3000° K or 6000° K. In cases of cataracts, by way of example, it is typically advantageous to illuminate with a correlated color temperature of 3000° K, since what is referred to as a red reflex, which illuminates the lens, can thus be produced. Said red reflex is brought about by a reddish to orange reflection of the illumination light at the retina. It is therefore advantageous if the light has a high proportion of red, which is the case for a low correlated color temperature. However, if the intention is to render for example opacities in the cornea, in the vitreous body or in the lens or scars in the cornea visible, it is advantageous to use light with a color temperature of about 6000° K, which has higher blue proportions (which undergo more pronounced scattering than the red proportion) than the light with a correlated color temperature of 3000° K. In a cataract operation, the scattering would have a negative effect however, which is why light with a correlated color temperature of about 6000° K is not usually used in such operations.
The surgical microscopes described in U.S. Pat. No. 7,443,579 B2 and DE 10 2007 026 044 B3 have spectral filters in their illumination apparatuses for changing the color temperature of the light emitted by the white-light source, which spectral filters can be inserted in the illumination beam path in order to convert for example the light of a xenon gas discharge lamp with a correlated color temperature of about 6000° K to light with a correlated color temperature of 3000° K, which corresponds to the light of a halogen incandescent lamp. If the red reflex is intended to be used, the spectral filter is inserted in the illumination beam path in order to convert the light from the xenon gas discharge lamp. Patent specification U.S. Pat. No. 7,443,579 B2 moreover describes the use of a halogen incandescent lamp together with a filter, wherein the filter then converts the light with a correlated color temperature of about 3000° K emitted by the halogen incandescent lamp into light with a correlated color temperature of about 6000° K.
Converting light of a first color temperature into light of a second color temperature, however, always entails a loss in intensity due to the wavelength components which are filtered out of the initial emission for conversion purposes. For example, if the intention is to use a xenon gas discharge lamp for illumination with a correlated color temperature of 3000° K, filtering out the corresponding spectral components reduces the illuminance, which can in principle be compensated for by corresponding readjustment of the lamp output. Such an adjustment, however, requires a correspondingly complex electronic arrangement and is not entirely without problems in respect of safety aspects either. If, for example, a transition is made during an eye exam or eye operation from an illumination with a correlated color temperature of 6000° K to an illumination with a correlated color temperature of 3000° K by way of insertion of a filter, and the lamp output is adjusted up in order to compensate for the loss in illuminance, it is necessary for the sake of patient health to ensure that the output of the xenon gas discharge lamp is reduced again as soon as the filter is taken out of the beam path again. This, too, necessitates a correspondingly complex electronic arrangement.
It is therefore an object of the present invention to provide an optical observation unit which enables a switching between illumination light with a first color temperature to illumination light with a second color temperature and in the process solves the abovementioned difficulties by simple means.
It is a further object of the invention to provide an advantageous method for ensuring an unchanging illuminance when changing the color temperature of the illumination by inserting or removing a spectral filter in or from the illumination beam path of an optical observation unit.